In the known method of German Patent Document DE-OS 23 57 210, which is the starting point of the invention, when the installation has been run in, defined extents of orientation of the plastic molecules in the axial and peripheral directions of the tubes can be achieved, for example, an axial orientation of about 20% and a peripheral orientation of more than 30%.
This stretching considerably improves the mechanical properties and, for the given mechanical properties, saves considerable material. The basic idea behind the steps of the earlier method is that a defined orientation or stretching in a plastic tube produced in an installation as described above always requires a particular temperature distribution in the tube wall which has to be stretched.
In the known process a homogeneous temperature distribution is provided in the tube wall to be stretched. This is usually a simple matter to achieve. For example the temperature adjustment is effected internally and externally in the region of the prolonged run-on part.
For inner temperature control, a heat exchange medium of an appropriate temperature is passed through the run-on part of the mandrel. For external temperature control, the plastic tube as a whole is passed through an appropriate bath the bath receptacle possibly being subdivided into a number of chambers at different temperatures.
If the hydraulic lubricant is introduced when the thermoplasticized plastic material is already in the quasi-liquid range, a drag flow of lubricant starts. This flow between the run-on cylinder and the inner wall of the tube further promotes heat transfer. Another effect is that all the volume elements of the tube to be stretched move at the same speed (without any internal transfer of material), thereby enhancing the defined temperature distribution and the maintenance thereof in the tube to be stretched.
The tube thus prepared, with its defined and very homogeneous temperature distribution, experiences a stretching or orientation of the plastic molecules substantially without sliding friction between the tube inner wall and the mandrel since the movement always take place on the lubricant which is in the form of a drag wedge of lubricating medium.
Surprisingly, the tube entrains this wedge provided that the expansion and the cooling do not occur too abruptly. The wedge forms, in longitudinal section, a skew plane and a hydraulic cushion. The wedge is prevented from detaching in the transition zone between the cylindrical run-on part and the conical expanding part. The drag flow of lubricant itself continues as far a the cylindrical draw-off part of the mandrel.
The formation of a lubricant drag flow also requires a certain quantity of the lubricant introduced. The hydraulic lubricant should be present in a substantially pressureless state at the entry. This applies particularly to entry in the quasi-liquid range. The hydraulic lubricant is therefore dragged along but is not forced in between the tube and the mandrel. This does not exclude the lubricant from being forced into the corresponding feed ducts to overcome friction losses therein.
In the known process the effect of the hydraulic lubricant is a hydrodynamic effect and not a hydrostatic effect. As previously mentioned, the plastic tube can be subjected to temperature control from the interior to the exterior in the region of the prolonged run-on part. The length thereof is such that this temperature control continues until a sufficiently defined and homogeneous temperature distribution is achieved.
Near the conical expanding part, the tube is exposed substantially to the stationary air. However, a liquid coolant can be used for cooling in this zone. In any case the tube is cooled intensively in the region of the draw-off part so that the thickness of the lubricant wedge is reduced considerably. This cooling produces considerable shrinkage of the tube. The shrinkage creates a gland effect and prevents the stretched tube from entraining any substantial quantity of lubricant. The advantage, therefore, is that the tubes can be stretched from thermoplastic materials biaxially in a very defined manner and with accurately predetermined and very substantial degrees of stretching, the molecules experiencing the biaxial orientation descried. Also, the inner diameter of the tube expanded by stretching is sized very accurately. However, the extents of orientation cannot very readily be adjusted in operation in the known process. Also, uncontrolled deviations from the programmed values may occur.